Wavelength Division Multiplexing (WDM) has been put into practical use as one technique for achieving large-capacity optical communications. In a WDM transmission system, a WDM optical signal in which a plurality of wavelength channels are multiplexed is transmitted via an optical fiber link. That is, a plurality of optical signals of different wavelengths are multiplexed and transmitted in the WDM transmission system.
A reconfigurable optical add/drop multiplexer (ROADM) is provided in each node in the WDM transmission system. The ROADM may drop an optical signal of a desired wavelength channel from a WDM optical signal. The ROADM may add an optical signal to an empty channel of a WDM optical signal.
FIG. 1 illustrates an example of a WDM transmission system. In the example illustrated in FIG. 1, an optical transmission device provided in each node is a reconfigurable optical add/drop multiplexer. The nodes are connected by optical fiber links.
A reconfigurable optical add/drop multiplexer (ROADM) 100 includes a pre-amplifier (pre-AMP) 110, a wavelength selective switch (WSS) 120, and a post-amplifier (post-AMP) 130. The pre-amplifier 110 amplifies a received WDM optical signal. The wavelength selective switch 120 drops an optical signal of a specified wavelength from the WDM optical signal amplified by the pre-amplifier 110. Further, the wavelength selective switch 120 may add an optical signal to an empty channel of the WDM optical signal. The wavelength selective switch 120 may adjust an optical power of each wavelength channel to a target level. The post-amplifier 130 amplifies the WDM optical signal output from the wavelength selective switch 120. Then, the reconfigurable optical add/drop multiplexer 100 transmits the WDM optical signal amplified by the post-amplifier 130 to an adjacent node.
The reconfigurable optical add/drop multiplexer 100 includes functions that respectively control gains of the pre-amplifier 110 and the post-amplifier 130. FIG. 1 illustrates the function that controls a gain of the pre-amplifier 110. An optical power monitor (PD) 111 detects an output optical power of the pre-amplifier 110. An amplifier controller 112 controls the gain of the pre-amplifier 110 such that the output optical power of the pre-amplifier 110 approaches a target level. This function maintains the output optical power of the pre-amplifier 110 at the target level.
An optical amplifier that can compensate for a sudden change in a power level of received light has been proposed (for example, Japanese Laid-open Patent Publication No. 2010-123698). Further, a method for setting a gain of an optical amplifier provided in a relay station of an optical transmission system has been proposed (for example, Japanese Laid-open Patent Publication No. 2006-33412).
As described above, the reconfigurable optical add/drop multiplexer 100 includes a function that controls an optical power of a WDM optical signal at a target level. However, when each reconfigurable optical add/drop multiplexer 100 controls the gain of the pre-amplifier 110 (or the post-amplifier 130) individually, the optical power of the WDM optical signal may be controlled improperly.
FIG. 2 illustrates an example in which an optical power of a WDM optical signal is controlled improperly. In the example illustrated in FIG. 2, a WDM optical signal is transmitted from a ROADM #1 to a ROADM #2. It is assumed that, before a time T1, an output optical power of the pre-amplifier 110 in the ROADM #1 is lower than a target level by 2 dB, and an output optical power of the pre-amplifier 110 in the ROADM #2 is lower than a target level by 3 dB.
In this case, a gain control is performed in the ROADM #1 such that the output optical power of the pre-amplifier 110 becomes higher by 2 dB. In the ROADM #2, a gain control is performed such that the output optical power of the pre-amplifier 110 is higher by 3 dB. However, in a WDM transmission system in which each reconfigurable optical add/drop multiplexer 100 operates independently, a gain control may be performed in the ROADM #1 and the ROADM #2 at the same time. In the example illustrated in FIG. 2, a gain control is performed at the time T1 in the ROADM #1 and the ROADM #2. In this case, the output optical power of the pre-amplifier 110 in the ROADM #2 is higher than the target level by 2 dB. In other words, an overshooting of an optical power occurs. When a WDM optical signal needs to pass through many ROADMs in a system, overshooting or undershooting may occur in a plurality of nodes and a communication state may become unstable.
This problem may be solved, for example, by correcting a gain of the pre-amplifier 110 more slowly. However, it is not preferable that the time needed for an optical power of a WDM optical signal to approach a target level be long.
The problem in which an optical power of a WDM optical signal is controlled improperly occurs not only in the control of the pre-amplifier 110. That is, this problem may also occur in the control of the wavelength selective switch 120 or the post-amplifier 130.